The present invention relates to a new kind of a method of producing pulp by using chlorine-free chemicals. The invention is especially related to ozone bleaching of pulp without preceding removal of heavy metals.
Different regulations and marketing requirements have set higher and higher demands on producers of kraft pulp to decrease or completely eliminate organic chlorine compounds in pulp products and effluents from bleaching. In order to be able to fulfill these demands the use of chlorine gas or any other compound containing chlorine (e.g. chlorine dioxide) should be avoided. When avoiding the use of chlorine-based bleaching chemicals, it is extremely difficult to obtain the desired brightness especially if the manufactured pulp has been given acceptable strength requirements. Consequently, lignin must be removed, for example, with oxygen. By using multi-stage oxygen delignification advantages have been achieved in delignification and in selectivity, especially when chelating is added to limit the amount of harmful metallic ions, and especially when between stage washing is included in the process (see U.S. Pat. No. 4,946,556). Practical hindrances, however, restrict both the delignification ad the quality of the pulp produced merely by oxygen delignification, especially if followed by an ozone bleaching stage.
So far a common and typical purpose of chlorine-free bleaching methods has been to remove heavy metals from the pulp as completely as possible prior to ozone treatment, since heavy metals are known to destroy ozone as taught by EP-A-0 512 590. Typical bleaching sequences by which pulp has earlier been bleached, are, for example, OOAZEZPZ, OAZEZPZ, OOAZEZP and OAZEZP. These sequences thus include one or by which pulp has earlier been bleached, are, for example, OOAZEZPZ, OAZEZPZ, OOAZEZP and OAZEZP. These sequences thus include one or more oxygen bleaching stages (O), an A-stage (acid washing), an ozone stage (Z), an extraction stage (E), a second ozone stage (Z) and a peroxide stage (P), and possibly a third ozone stage (Z). In the acid stage (A) prior to the first ozone stage heavy metals are removed, which are flushed away, when a portion of the wash filtrate is removed. The extraction stage (E) may be an oxidizing peroxide extraction stage or a conventional oxidizing extraction stage. The ozone bleaching stages are preferably carried out with pulp having the consistency of about 5-18%.
It is characteristic of the above mentioned sequences that they include at least five washing stages, in other words bleaching stages alternate with washing stages, i.e. washers, by which the chemicals separated from the fibers as reaction products or otherwise in each bleaching stage are removed from the suspension. Since the washers form a considerable part of the investment costs in a bleaching plant, the number of the washers should, of course, be limited as much as possible, if it is only possible without risking the quality of the final product.
In the same connection chemicals necessary for some bleaching reactions, such as magnesium (Mg), can also be removed from the pulp, which requires addition of magnesium subsequent to the ozone treatment.
Other multi-stage bleaching sequences have been described for instance in EP patent applications EP-A-0 426 652, EP-A-0 512 978 and EP-A-0 402 335.
EP-A-0 426 652 discloses several bleaching sequences where ozone has been used in combination with peroxide and oxygen. For instance, sequences like EOP-Z-PE-Z-PE, EOP-Z-PE, O-Z-EO-Z-P, O-Z-EOP-Z-P and O-Z-EO-Z-D were discussed. However, the publication does not take into account the presence of heavy metals in the pulp in spite of the fact that at least one of the examples discusses bleaching of kraft pulp by means of a peroxide containing compound.
EP-A-0 512 978 discloses a multi-stage bleaching process using both ozone and peroxide for bleaching pulp. The document does not pay any attention to the removal of heavy metals from pulp before bleaching with peroxide.
EP-A-0 402 335 discloses a process for bleaching lignocellulose-containing pulps by means of peroxide containing substance. The main teaching of the document is to alter the trace metal profile of the pulp by means of a separate treatment with a complexing agent. In other words, the document teaches the importance of treating pulp prior to a peroxide stage in a separate metal removing stage. The document further teaches that the treatment comprises a washing stage between the treatment with complex formers and the peroxide stage.
On the other hand, the previously used chlorine has also prevented a screening stage subsequent to bleaching from being combined with the bleaching plant, because in chlorine bleaching the screening and/or vortex cleaning treatment of pulp would lead to serious corrosion problems. Thus it has been necessary to carry out the vortex cleaning and/or screening as a separate stage. A conventional bleaching+screening plant has thus included four or five bleaching stages and one screening and/or vortex cleaning stage, which the washing stage separates from each other. The present invention compresses these five to six stages into three stages and thus almost halves the investment costs of a bleaching plant and a screening plant.
By utilizing the present invention it is possible to eliminate the above mentioned disadvantages occuring with the removal of heavy metals and the omission of screening in the prior art technique. At the same time a bleaching plant is provided, which includes only three washing stages.
It is characteristic of the method in accordance with the present invention that pulp is bleached with a sequence beginning with ozone and without the removal of heavy metals preceding the ozone stage.
It is characteristic of another embodiment of the method in accordance with the present invention that pulp is bleached with a three-stage sequence (ZT)(EOP)(ZP) without the removal of heavy metals preceding the sequence, in which
(ZT) refers to a bleaching stage with ozone, which also includes treatment of heavy metals and which stage is followed by washing and/or thickening,
(EOP) refers to a bleaching stage with peroxide or oxygen and peroxide in alkali conditions and the stage is followed by washing and/or thickening,
(ZP) refers to bleaching stage with ozone and peroxide without a between stage washing and which stage is followed by washing and/or thickening.
In each of the stages (ZT), (EOP), and (ZP) there is no interstage washing.